The field of the invention relates to signal detection with hysteresis.
Conversion of an analog signal into a two-state signal is necessary before digital processing. For example, in automobile applications, a sensor coupled to an automobile's drivetrain provides an analog signal having a frequency related to vehicular speed. After conversion to a digital signal, a microcomputer detects the time interval of each cycle and computes vehicular speed by conventional algorithms. A problem encountered is that the sensor signal has a high degree of noise due to the physical environment of the sensor and appropriate connecting cables. This noise may be erroneously detected as a signal transition thereby adversely affecting any resulting computations.
It is known to use a low pass filter between the sensor signal and microcomputer to improve signal to noise ratio as disclosed in U.S. Pat. No. 4,668,875 issued to Miyazaki et al on 26 May 1987. However, dependent upon the particular application, a low pass filter may not provide sufficient signal to noise ratio over a wide range of signal frequencies.
An approach to improve signal to noise ratio is to use multiple transistors configured as a differential amplifier for providing a comparator with hysteresis about an external reference signal. When the input signal exceeds the reference signal by the hysteresis value, a logic "1" output is provided. Conversely, when the input signal falls below the reference signal by the hysteresis value, a logic "0" output is provided. One disadvantage of this approach is that noise components may cause false triggering at either the upper or lower threshold values. False triggering at the threshold values may be reduced by connecting a latching circuit to the differential amplifier as disclosed in U.S. Pat. No. 4,642,484 issued to Skovmand et al on 10 Feb. 1987.
A disadvantage of the above approaches is the circuit complexity of using comparators or differential amplifiers and latching circuitry. In applications such as automotive applications, where the size and resulting interconnections of electrical circuitry are constrained by a host module, these disadvantages may be prohibitive.